Conventionally, in applying liners to corrugated medium the first liner is applied at the single facer to form a single faced board and this is achieved by applying adhesive to the flute tips while the medium is contained on the corrugating roll and then applying the liner under pressure and heat. The combination of heat from the corrugating roll and pressure roll and mechanical pressure itself forms the first liner to medium bond. Subsequently, the single face board is passed to a double facer or double backer where adhesive is applied to flute tips and the board passed over a series of steam heated platens to bond the second liner. Pressure is also applied by a transport belt on top of the board and a series of small rollers riding on that belt. Travel of the double face board through the double facer or double backer is assisted by sandwich belts after the steam heated platens.
The size of the plant and the length of the production floor is quite long in conventional plant for producing corrugated board. The heating platens are of extensive length to ensure sufficient curing of the adhesive bond as the board passes over the platens at speeds of up to 300 to 400 meters per minute.
Another characteristic of conventional corrugated board is that of "pressure lines" which result from the pressure applied by the pressure roll at the single facer. This is compounded by "washboarding" which is the effect of the adhesive, when drying, drawing the liner out of its linearity. These effects make the board unsuitable for high quality printing on that side.